I. Field of the Invention
This invention relates generally to a device for stimulating cardiac tissue, and more particularly relates to an implantable cardiac rhythm management device and method for automatically adjusting the evoked response detection threshold. Without limitation, during automatic capture verification the cardiac rhythm management device receives and digitizes an electrocardiogram signal of the patient's heart and adjusts the evoked response detection threshold dependant upon modulation or changes in the amplitude of the evoked response from beat-to-beat. Several factors influence the modulation of the amplitude associated with the evoked response including respiration of the patient. The respiration of the patient affects the amplitude of the evoked response for each R-wave, increasing and decreasing the amplitude in a cyclic fashion. Other factors further modulate the amplitude of the evoked response for each R-wave. The present invention accordingly adjusts the evoked response detection threshold in relation to the modulation of the amplitude of the evoked response.
II. Discussion of the Prior Art
For the most part, prior art implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers and cardiac defibrillators, have sense amplifier circuits for amplifying and filtering electrogram signals detected by electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. Typically, the signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a source or reference potential. Only when an electrogram signal from the sense amplifier exceeds the preprogrammed reference potential threshold will it be treated as a cardiac paced or sensed beat. The source reference potential may be referred to as an evoked response detection threshold.
Typically, the reference potential threshold is set at a fixed amount that is expected to exceed the maximum amplitude of artifact. If, however, the threshold is not set high enough, then artifact may result in malsensing. Further, if the reference potential threshold is set too high, then the amplitude associated with an R-wave may not be sufficient to trigger the reference potential threshold.
The inventors of the present invention have determined that several factors influence the amplitude associated with R-waves. For example, respiration of the patient fluctuates the amplitude of the evoked response in a cyclic fashion increasing and decreasing over several beats. This fluctuation makes it even less desirable for the reference potential threshold to be set at a fixed amount. Other factors, described in greater detail below, affect the amplitude of the evoked response. Thus, a need exists for a cardiac rhythm management device that automatically adjusts the evoked response detection threshold in conjunction with fluctuations in the amplitude associated with a particular R-wave, taking into account the modulation of the amplitude corresponding to the evoked response for each R-wave. The present invention meets these and other needs that will become apparent from a review of the description of the present invention.